Before storming the building, the leader of the SWAT team kissed the pendant his wife had given him for luck and tucked it back under his shirt. First through the door, he saw the gun for only an instant before he felt the bullet punch him hard in the chest. He heard more gunfire as he fell back, the room spinning around him.

Thanks to his ballistic vest, he was up and talking with fellow officers five minutes later. Though he would feel some soreness from the impact for several days, he had survived a direct hit from a handgun fired at close range.

Before examining modern body armor, let's consider its history and evolution. The origin of armor is prehistoric, likely dating to the time when humans began using weapons against one another in an organized fashion. The earliest armor was probably animal skins, particularly from animals with especially tough hides. There is evidence that rhinoceros skin was used as armor thousands of years ago. Animal hides were succeeded by coverings made from other natural materials, such as bone and wood. Such armor would have been at least partially effective in stopping primitive stone hand weapons and arrowheads.

But the evolution of armor has been driven throughout history by increasingly lethal weaponry. When the Bronze Age began about 5000 years ago, bronze weapons demanded bronze armor. Similarly, when iron weapons appeared a couple thousand years later, so did crude iron armor. Heavy plates of iron to cover the front (breast plate) and back (back plate) of the torso were the most common.

The next major leap in the evolution of armor was the development of mail (sometimes called chainmail). Mail armor was constructed of tiny rings of iron interlocked together by welding or riveting to form a continuous latticework. Both round and flat rings were used to produce mail, the former being more effective against swords, the latter preferable against arrowheads.

More complete body suits were possible with mail. However, such a suit might require tens of thousands of iron rings, making it very labor intensive and expensive to produce. Nonetheless, by 1000 A.D. mail was the armor of choice for those who could afford it.

Over time, small iron plates were devised to supplement mail and protect vulnerable areas such as the knee and groin. By the fifteenth century this had evolved into the full plate body armor that is likely to leap to mind when one thinks of medieval knights. Early plate armor was iron, but in response to the introduction of firearms onto the battlefield in the sixteenth century, manufacturers began to harden the iron by the addition of alloying elements and employing heat treatments to create a carburized layer, essentially an early form of steel.

Steel plate was relatively easy to form into various shapes, resulting in the medieval knight covered head to toe in armor. Mail continued to be used alone but also in conjunction with plate armor to protect joints such as knees and elbows where steel plates were joined.

While armor was bulky and cumbersome, it was perhaps not as heavy as myth would have it. A full suit of plate armor weighed 50-70 pounds, no more than the weight carried by a well-provisioned modern soldier.

Plate armor, though not as labor intensive to produce as mail, was also beyond the means of most. Only the nobility, wealthy landowners, and professional soldiers could have afforded such an investment.

The continued development of firearms eventually spelled the end of plate armor. Early firearms were not capable of penetrating steel plate, and knights retained their immunity from attack for some time. But by the 1700s, muskets of sufficient power to penetrate plate armor rendered it obsolete for battle. Nobility continued to employ ornate armor for ceremonial purposes.

However, the story of body armor does not end there. Technology has provided effective armor for police and security personnel as well as for the military. Modern body armor may be divided into two broad categories: soft and hard.

Soft armor is relatively lightweight and comfortable. It is typically constructed of Kevlar, very strong synthetic fibers wound tightly like a rope. Kevlar fibers are then woven together into a dense net or mesh. The resultant material is glued between plastic sheets and sewn onto a cloth vest. The vest must protect the wearer against both the penetration of a bullet and the impact or "punch" it delivers. It does this by "catching" the bullet in the Kevlar net, stopping it and causing it to deform or spread out on impact rather than penetrate. Simultaneously, the impact pulls on the mesh structure, distributing the force over the entire torso.

There is no such thing as a bullet-proof vest; the ultimate effectiveness of body armor depends on its construction and the caliber and velocity of the bullet. Kevlar vests are less effective against knife attacks than bullets, as they are not designed to arrest the penetration of a sharp point. Kevlar vests are properly called "ballistic vests" to emphasize this distinction.

But soft armor such as Kevlar does strike a perfect balance between protection and mobility for police. It is sufficiently light and non-constraining to allow an officer to chase a suspect or grapple hand-to-hand, while providing pretty good defense against bullets.

Warfare, however, often requires more robust protection. Soft armor cannot reliably stop shrapnel from fragmentation grenades, artillery shells, and similar threats faced by modern soldiers. These projectiles are small, sharp, and may travel at high velocity. They are also hard metal, such as steel, and will not deform on impact like a soft lead bullet. Thus military personnel often resort to hard body armor, constructed of overlapping steel, plastic, or ceramic plates encased in a cloth vest. In fact, it is reminiscent of medieval plate armor, but updated with space-age materials. The best hard armor utilizes disks of ceramic materials such as aluminum oxide or silicon carbide, overlapped like the scales of a fish.

Hard armor works on the same principle as that worn by the knights of old; it makes no effort to absorb or deform the projectile. Instead, it is hard enough to simply deflect it. There are also soft armor vests on the market built with large pockets into which steel or ceramic plates can be slid to quickly upgrade to hard armor.

While hard body armor offers outstanding protection, it is heavy, cumbersome, and hot. The added discomfort and loss of mobility is tolerable only for those at very high risk of attack. In addition to soldiers in a war zone, SWAT teams, elite bodyguards, and similar operatives may utilize hard armor as the occasion warrants.